This invention relates to a method of and apparatus for detecting the presence of corrosion damage, and more particularly the position and degree of such damage, in hollow pipes or tubes such as those employed in heat exchangers.
In the field of petroleum refining, for example, heat exchangers having a number of pipes or tubes (hereinafter referred to collectively as "pipe") are utilized. This pipe often is exposed to a corrosive atmosphere, making it necessary to inspect them for security and remaining service life.
One method of detecting the degree of corrosion damage in such pipe known as the eddy current type flaw detecting method is extensively employed for austenite stainless steel pipes and brass pipes. This method is described in "Ishikawajima-Harima Engineering Review", Vol. 18, No. 1 (January 1978), pp. 38-41. The eddy current method uses an exciting coil and a detecting coil; flaws in a pipe are detected according to a pulse signal outputted by the detecting coil, or in variations in the impedance thereof. However, the system is disadvantageous in that the output signal does not correspond to the depth of a portion of the pipe damaged by corrosion (hereinafter referred to as "a corrosion portion"). In addition, if the pipe to be inspected is made of a magnetic material, e.g., steel, it must first be magnetically saturated, usually by inserting a coil carrying an electric current into the tubes, which involves considerable difficulty.
A method of measuring the wall thickness of a pipe using radiant rays (e.g., from a radio isotope) is known in the art as the radiograph inspection technique. The radiograph technique is not effective for measuring the degree of corrosion of the inside of the tube. And it is impossible thoroughly to inspect bundled pipe such as might be found in a heat exchanger using the radiograph technique. That is because the measurement of the wall thickness for such pipe can be carried out in one direction only, due to difficulty in positioning the film used in the radiograph technique when the tubes are arranged in close proximity to one another. In addition, fiberscopes have been employed to inspect the inside of pipe, a method which is low in efficiency and which is liable to miss corrosion portions.
In view of the deficiencies of the foregoing, the art has sought other methods more positively and reliably to detect and to measure the degree of corrosion damage in pipe. For instance, destructive sampling inspection in which a typical pipe is removed from the heat exchanger and inspected for damage, from which the corrosion damage to the remaining pipe can be estimated, has been used. However, it is apparent that such a sampling inspection method is based on estimation, is low in efficiency, and is commercially uneconomical. Methods of measuring the wall thickness of a pipe with ultrasonic waves and of directly measuring a flaw with a depth gauge also are known in the art. However, those methods are inefficient and incapable of highly accurate measurement.
Thus the art has sought a method of detecting the degree of corrosion damage which avoids the above-described difficulties accompanying prior methods. In one system, as shown in FIGS. 1 and 2, electrically conductive disks 1 and 2 separated by distance d are disposed perpendicular to the axis of a pipe 3 to be inspected, the circumferential surfaces of the disks being adjacent the inner wall of pipe 3. Pipe 3 has a corrosion portion 4 in its inner wall.
As shown in FIG. 2, disks 1 and 2 can be held by cylinder member 5 made of an insulating material, so that they can be moved longitudinally of the pipe while maintaining constant the distance d between them. For example, as shown in FIG. 1, disks 1 and 2 can be moved in the direction of the arrow from position (I) to position (II) in pipe 3. As shown in FIG. 2, insulating rings 6 and 7, each of which has an outside diameter larger than that of disks 1 and 2 but smaller than the inside diameter of pipe 3, are placed over the two end portions of cylinder member 5; compressed air supplied into pipe 3 moves disks 1 and 2 in the direction of arrow P, for example.
When disks 1 and 2 carry electrical charges opposite in polarity (i.e., one disk charged positively and one disk charged negatively), the lines of electric force between the disks are curved outwardly in the vicinity of the edges of the disks. Moreover, the dielectric constant of the medium through which these outwardly curved lines of force travel differs with the disks located at position (I) in FIG. 1, where there is no damage to pipe 3, from that at position (II), where corrosion portion 4 exists. This difference in dielectric constants occurs due to corrosion portion 4 in the inner wall of the pipe, and as a result the capacitance C.sub.x between disks 1 and 2 is different at position (I) from that at position (II).
The degree of corrosion damage can be detected, as shown in FIG. 3, using bridge circuit 10 formed with disks 1 and 2. A high frequency voltage (144 MHz, for instance) is applied to bridge circuit 10 by high frequency oscillator 11. The resulting unbalanced output voltage of bridge circuit 10 is rectified by rectifying diode 12, and the output of diode 12 is amplified by amplifier 13 and displayed on voltmeter 14. In FIG. 2, reference numeral 15 designates a lead wire or coaxial cable connected to oscillator 11 in FIG. 3, and reference numeral 16 designates a lead wire connected between amplifier 13 (see FIG. 3) and bridge circuit 10 (see FIG. 3), which may be built into the probe.
The values of reference capacitor C.sub.s and reference inductance coils L.sub.1 and L.sub.2 of bridge circuit 10 are selected such that the output of bridge 10 is at a minimum when the disks are not adjacent a corrosion portion, e.g., at the position (I) in FIG. 1. Thus, when disks 1 and 2 are moved to position (II), adjacent corrosion portion 4, the value of the static capacitance C.sub.x of the disks is changed as described above, increasing the unbalanced output of bridge circuit 10. The degree of variation of that output corresponds to the degree of corrosion damage. Using an analytical curve, e.g., a curve determined in advance from reference pipes relating readings of voltmeter 14 to the degree of corrosion damage, the degree of corrosion damage can be measured.
FIG. 4 shows another detecting device known to the art. Inductance coil 17 is provided instead of the capacitance between the disks in FIGS. 1-3. The magnetic lines of flux emanating from inductance coil 17 link the walls of pipe 3, which walls vary in magnetic permeability due to the presence of corrosion portion 4. In this case, the degree of corrosion damage is detected by the change in coil inductance caused by variations in the magnetic permeability of the pipe walls. Coil 17 is part of bridge circuit 10, analogous to bridge circuit 10 described above in connection with FIGS. 1-3 and shown diagrammatically in FIG. 3.